So-called "spinning disc" applicators for herbicides and other agents are known (see, for example, British Patent Specification No. 2131327). These applicators have a rotary distribution element or "disc" which is rotated at speeds of, for example, between 200 and 4000 rpm. The agent to be applied is fed to the surface of the disc and is ejected from the periphery of the disc by centrifugal force. Applicators of this type are capable of producing a well-defined spray pattern with an even droplet size under most conditions. The width of the spray pattern can be altered by changing the speed of rotation of the disc, and the spray density can be altered by changing the rate of flow of the agent to the disc. In the equipment of GB No. 2131327, a number of sockets are provided which communicate with restrictors of different sizes, and the rate of flow of the agent is changed by plugging a supply duct into the appropriate socket. A similar plug-and-socket system is used in one embodiment described in British Patent Specification No. 2155816, which also discloses another embodiment in which the flow rate is controlled by varying the width of an annular gap through which the agent flows to the disc. However, none of these constructions is capable of maintaining accurate control of the flow rate, while permitting easy adjustment.